In many commercial buildings, the heating-ventilation-and-air conditioning (HVAC) system utilizes both an economizer, to supply a mixture of outside air blended with some of the air returned from the building, and local variable air volume (VAV) apparatuses or boxes that supply conditioned air for each room (or series of rooms or a zone being served by a common supply duct). Economizers are primarily used to reduce the energy needed to condition the air supplied to the building. For example, when the outside air temperature is cooler than the return air, but higher than the desired supply temperature, economizers use more outside air in order to minimize the need to provide cooling from a machine. VAV boxes are used to vary the amount of air to that which is needed to each room (or zone), thus reducing the total amount of air used and energy used to condition and transport the air.
VAV boxes typically modulate air-flow rates as governed by the thermostat temperature reading between a minimum set-point and a maximum set-point, depending on the cooling demand of the occupied space. As the cooling load requirement for the room increases, the air-flow rate set-point increases until it reaches the maximum set-point. For heating, the air-flow rate is set to the minimum set-point value and the reheat valve position is changed according to the amount of heat required from the supply air. However, the minimum VAV air flow set-point has historically been set between 30 to 50% of the maximum air flow set-point.
The amount of supply air flow rate for each room typically needs to be high enough to ensure that ventilation and conditioning needs are met. The amount of ventilation being provided to a room is determined by the volume flow rate of outdoor air entering the room. For a VAV system utilizing an economizer, the supply air is made up of a mixture of return air (e.g., returned to an air handler unit (AHU) or air handler from rooms throughout the building) and outside air, and there might need to be significantly greater airflow than if only outside air was being supplied to the room.
Current VAV minimum airflow set points are specified considering the minimum outside air percentage designed into the AHU/economizer control. Thus, if the minimum outside air percentage is 20%, the minimum airflow set point for a VAV box is typically set to 5 times the outside air requirement for that VAV box.
Currently, the economizer dampers are controlled to achieve a desired mixed air temperature set point, TmixSP, in order to minimize the energy used to achieve desired level of dehumidification, as well as thermal conditioning (cooling as well as heating). For example, the TmixSP can be 55° F. during the summer, to ensure humidity is controlled.